1. Field of the Invention
This invention relates to a phase change optical recording medium and a method for its initialization (crystallization of its recording layer).
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change in reflectivity by the crystallographic change is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Gexe2x80x94Te system and Gexe2x80x94Sbxe2x80x94Te system which provide a substantial difference in reflectivity between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds have been investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb2 or IIb-IVb-Vb2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228. Among the chalcopyrite compounds, AgInTe2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992. In addition to the optical recording media of phase change type wherein chalcopyrite compounds are used, optical recording media of phase change type wherein AgSbTe2 phase is formed with the crystallization of the recording layer is disclosed in JP-A 267192/1992, 232779/1992, and 166268/1994.
When information is recorded on the optical recording medium of phase change type, the entire recording layer is first brought into crystalline state, and then, a laser beam of high power (recording power) is applied so that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallization temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallization temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
In the case of the phase change type optical recording media, recording layers are formed using vacuum deposition equipment and the as-deposited recording layers remain amorphous with low reflectivity. The recording layers must be crystallized by an operation generally known as initialization before the recording media can be utilized as rewritable media.
Initialization is carried out in various ways, for example, after a recording layer is formed on a substrate, by heating the substrate to the crystallization temperature of the recording layer for crystallization as disclosed in JP-A 3131/1990; illuminating a laser beam to the recording layer for crystallization, which method is called solid phase initialization, as disclosed in JP-A 366424/1992, 201734/1990 and 76027/1991; and high-frequency induction heating the medium. JP-A 98847/1990 proposes to heat a substrate during formation of a recording layer to thereby crystallize the recording layer. JP-A 5246/1990 discloses a method involving the steps of forming a first dielectric layer, forming a recording layer thereon, heating it for crystallization, and forming a second dielectric layer thereon.
However, the initialization of the medium one track at a time using the laser beam of small beam spot diameter as used in the recording takes a long time and is makes the productivity lower. Heating of the overall medium rejects the use of inexpensive resin substrates. That is, resin substrates can be distorted upon heating for initialization, causing errors in tracking. Under the circumstances, the use of a so-called bulk erasing is the only technique which is regarded commercially acceptable and currently used. The bulk eraser illuminates a beam from a high-power gas laser or semiconductor laser through a relatively large aperture stop for crystallizing a multiplicity of tracks altogether. Since the bulk eraser permits the recording layer to be locally heated, the substrate temperature is elevated to a little extent, enabling the use of less heat resistant resins as substrates.
Initialization of an optical recording disc with a bulk eraser, however, is a time-consuming process, and it takes several minutes just to initialize one optical recording disc. The process of initialization has been the rate-determining step in the production of the optical recording discs. Speedup of the initialization step is required for improving the production efficiency. The time required for the initialization by a bulk eraser may be reduced by using a high-power laser at a higher disc rotation rate, by increasing the feed rate simultaneously with the increase in the beam spot diameter, or by combining the both. In other words, the initialization step can be speeded up by increasing the area irradiated by the laser beam per unit time while retaining the irradiation power per unit area at the level necessary for the initialization. The laser output, however, can be increased only to a limited level.
In view of such situation, there is a demand for decrease in the laser power per unit area necessary for the initialization. Such decrease in the required laser power per unit area will enable to increase the initialization speed, and in addition, to elongate the life of the laser since the full powered operation of the laser can be avoided with a small sacrifice in the initialization speed.
In view of the situation as described above, an object of the present invention is to provide a phase change optical recording medium wherein the laser power per unit area necessary for the initialization is reduced, and a method wherein the initialization can be accomplished at a lower laser power.
Such objects are attained by the present invention as described in (1) to (7), below.
(1) An optical recording medium having a phase change recording layer satisfying the relations:
AIxe2x89xa68.0%,
and
CI/AIxe2x89xa73.0
when the medium is initialized with a light beam having a wavelength xcexI, and said recording layer exhibits a reflectivity AI in amorphous region and a reflectivity CI in crystalline region at said wavelength xcexI.
(2) An optical recording medium according to the above (1) satisfying the relation:
xcexI greater than xcexRW
when the light beam used in writing and/or reading has a wavelength xcexRW.
(3) An optical recording medium according to the above (2) satisfying the relation:
xcexIxe2x88x92xcexRWxe2x89xa7100 nm
(4) An optical recording medium according to the above (1) wherein the medium comprises a substrate, and a first dielectric layer, said recording layer, a second dielectric layer, and a reflective layer disposed on said substrate, and the medium satisfies the relation:
65 nm less than d1xe2x89xa6143 nm
when the first dielectric layer has a thickness d1.
(5) An optical recording medium according to the above (4) satisfying the relation:
71.5 nmxe2x89xa6d1xe2x89xa6143 nm
(6) A method for initializing an optical recording medium having a phase change recording layer wherein the medium is initialized with a light beam having a wavelength xcexI satisfying the relations:
AIxe2x89xa68.0%,
and
CI/AIxe2x89xa73.0
when said recording layer exhibits a reflectivity AI in amorphous region and a reflectivity CI in crystalline region at said wavelength xcexI.
(7) A method for initializing the optical recording medium according to the above (6) satisfying the relation:
xcexI greater than xcexRW
when the light beam used in writing and/or reading has a wavelength xcexRW.